pinetree_raccoon
Actually, I was curious about this and found there are more issues on Android affecting bubble notifications. GrapheneOS devs have an issue open for a related problem. The related issue is with Android on Google's end, which is called an "upstream" issue. This means it affects more than just GrapheneOS. When Google issues a fix, it should be merged in to GrapheneOS. Hopefully this semi-related issue's fix will fix your problem too.